Registering coherent change detection products associated with large image sets and long capture intervals

ABSTRACT

A set of co-registered coherent change detection (CCD) products is produced from a set of temporally separated synthetic aperture radar (SAR) images of a target scene. A plurality of transformations are determined, which transformations are respectively for transforming a plurality of the SAR images to a predetermined image coordinate system. The transformations are used to create, from a set of CCD products produced from the set of SAR images, a corresponding set of co-registered CCD products.

This invention was developed under Contract DE-AC04-94AL85000 betweenSandia Corporation and the U.S. Department of Energy. The U.S.Government has certain rights in this invention.

FIELD

The present work relates generally to imaging with synthetic apertureradar (SAR) and, more particularly, to co-registration of coherentchange detection (CCD) products.

BACKGROUND

Coherent change detection (CCD) offers the capability for detectingchanges that occur in a target scene between SAR imaging passes. Asequence of SAR imaging passes produces a sequence of complex SAR imagesthat respectively correspond to the imaging passes. The CCD processproduces a set of CCD products, each of which represents a comparison ofa pair of SAR images of the target scene that have been captured duringtime-wise adjacent airborne imaging passes. The creation of each CCDproduct involves a sub-pixel co-registration of the associated pair ofSAR images. The set of CCD products is then registered to a commoncoordinate system, resulting in a set of co-registered CCD products foruse in producing statistical models about the target scene, on apixel-by-pixel basis. Traditionally, however, for a large set of SARimages taken across a long span of time, it has been difficult toproduce a co-registered set of CCD products with sufficient accuracy topermit collection of useful statistics.

In particular, extending the aforementioned pair-wise, sub-pixelco-registration approach across many images presents some problems.First, every time an image is transformed a small amount of error isintroduced. Second, creating a good CCD product requires very accuratesub-pixel registration. Accurate sub-pixel registration cannot beachieved with complex SAR images that are significantly separated intime, which leads to poor quality CCD products. These problems make itdifficult to use simple solutions such as registering all of the CCDproducts to a common coordinate system.

It is desirable in view of the foregoing to provide for producing a setof co-registered CCD products that is suitable for statistical analysisand avoids problems such as those associated with longer image sequencesand increased time between image passes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates conventional use of sub-pixelregistration techniques to produce a set of CCD products from a set ofSAR images.

FIGS. 2-4 illustrate operations used to produce a set of co-registeredCCD products according to example embodiments of the present work.

FIG. 5 diagrammatically illustrates an apparatus configured to produce aset of to co-registered CCD products according to example embodiments ofthe present work.

DETAILED DESCRIPTION

Example embodiments of the present work provide for co-registering alarge set of SAR CCD products that span a significant amount of time.The present work exploits the fact that conventional techniques forproducing a CCD product from a pair of complex SAR images use extremelyaccurate sub-pixel registration between the pair of complex SAR images.According to example embodiments of the present work, transformationsproduced by co-registration of the complex SAR images are used toco-register the corresponding CCD products. Some embodiments maintainthe best image registration possible for CCD products while reducing thenumber of image transformations, producing high quality CCD productswith minimal error introduction.

Example embodiments of the present work create a co-registered set ofCCD products corresponding to a time interval of SAR image capture byfirst producing a set is of time-ordered CCD products according to aconventional technique. In particular, the raw complex SAR images areordered with respect to the time they were taken, and indexed as SAR i,for i=1, 2, . . . , as shown at 11 in FIG. 1. The SAR images shown at 11are captured sequentially during an image capture interval. Startingwith the second image in the sequence, SAR 2, each image is sub-pixelregistered to the immediately preceding image, as shown at 13 in FIG. 1,to create a corresponding CCD product designated by CCD(i, i+1), asshown at 15 in FIG. 1. The aforementioned sub-pixel registration is alsodesignated herein as fine registration. Techniques for capturing asequence of SAR images are known in the art, as are techniques forperforming sub-pixel registration between sequentially adjacent pairs ofthe captured SAR images to produce corresponding CCD products.

According to example embodiments of the present work, the set of complexSAR images 11 is processed to co-register the images. The process ofco-registering the set of complex SAR images is also referred to hereinas coarse registration. The coarse registration process producestransformations for transforming the SAR images to a predetermined imagecoordinate system. These transformations produced during the coarseregistration are used to co-register the set of CCD products 15. In someembodiments of co-registering the SAR images, a base image is initiallyselected from the set of SAR images 11. Some embodiments select as thebase image the image closest in time to the middle of the SAR imagecapture interval encompassed by the set of SAR images 11. By using thisbase image, error propagation may be minimized, because all images maybe transformed directly into the coordinate system of the base image.

Each SAR image is registered to the base image in conventional fashion,using a corresponding transformation into the coordinate system of thebase image. For a given image SAR i at 11, the correspondingtransformation T_(i) that transforms that image to the coordinate systemof the base image is applied to the CCD product that corresponds toimage SAR i, namely, CCD(i, i+1). The result is a set of co-registeredCCD products.

The longer the SAR image capture interval, the more likely it is thatattempts to register some of the captured SAR images to the base imagewill fail, due to changes in the scene being imaged. Over a relativelylong image capture interval, these changes can be drastic and encompassthe entire scene. For example, manmade structures may be added orremoved, trees and other plants may grow, die, or be cut down, or fieldscan be tilled, planted, or harvested. Some embodiments use anintermediate base image when a registration failure occurs. Aregistration failure is declared for a SAR image if, during theconventional process of registering that image to the base image, thenumber of tie points detected is below a predetermined threshold. If aregistration failure occurs, the last SAR image that was successfullyregistered to the current base image is selected as a new, intermediatebase image. The image that failed to register, and subsequentlyprocessed images, are then registered to the intermediate base image.The corresponding CCD product will therefore not be transformed directlyto the coordinate system of the original base image, but rather will betransformed according to a compound transformation wherein theimage-to-intermediate base transformation is applied to the CCD product,followed by the intermediate base-to-original base transformation. Someembodiments determine a suitable tie point threshold for failuredeclarations based on empirical observations under anticipated operatingconditions.

The use of an intermediate base image is readily extended to one or moreadditional intermediate base images. That is, if a registration failureoccurs relative to the aforementioned intermediate base, someembodiments select a further intermediate base in the same manner as theprevious intermediate base was selected. This further to intermediatebase is registered to the previous intermediate base, which is in turnregistered to the original base. One or more still further intermediatebase images may be selected as necessary. The use of one or moreintermediate base images to register a given image SAR i to thecoordinate system of the original base image means that thecorresponding CCD product, CCD(i, i+1), will have associated therewith aset of transformations, also referred to herein as a compoundtransformation. This compound transformation is applied to the CCDproduct, thereby co-registering that CCD product to the original baseimage. For example, if two intermediate bases are required to registeran image SAR i to the original base image, this results in a compoundtransformation that includes the following three transformations: (1) atransformation of SAR i to the coordinate system of the secondintermediate base image; (2) a transformation of the second intermediatebase image to the coordinate system of the first intermediate baseimage; and (3) a transformation of the first intermediate base image tothe coordinate system of the original base image.

Some embodiments are capable of producing a set of co-registered CCDproducts corresponding to a sequence of 350 SAR images captured over asix-month image capture interval. A limiting factor is the amount ofmajor permanent changes (e.g., demolition of a building) that occur inthe scene. As described above, the use of intermediate bases helpsameliorate these types of limitations.

FIG. 2 illustrates operations performed according to example embodimentsof the present work. The SAR images (see also 11 in FIG. 1) are obtainedat 21. At 22, CCD products are produced from the SAR images (see also 13in FIG. 1). Also at 22, transformations for transforming the SAR imagesto the coordinate system of an original base image are produced. At 23,each CCD product, CCD(i, i+1), has applied thereto the transformation(direct or compound) that transforms the associated image, SAR i, to thecoordinate system of the original base image, thereby producing thedesired set of co-registered CCD products.

FIG. 3 illustrates operations used to produce transformations (see also22 in FIG. 2) according to example embodiments of the present work. At31, an original base SAR image is selected. At 32, the identity of theselected base SAR image is recorded. At 33, the next SAR image isregistered to the base SAR image, and information indicative of thetransformation used to accomplish the registration is saved. If it isdetermined at 34 that the registration at 33 was successful, thenregistration of the next SAR image commences at 33, unless it isdetermined at 36 that all SAR images are registered, in which caseoperation is completed. If it is determined at 34 that the registrationat 33 has failed (for example, because the number of tie points detectedis below a threshold), then the last successfully-registered SAR imageis selected at 35 as an intermediate base image. The identity of thisintermediate base image is then recorded at 32, and the registrationprocess continues at 33.

FIG. 4 illustrates operations for transforming a set of CCD products toproduce a set of co-registered CCD products (see also 23 in FIG. 2)according to example embodiments of the present work. A CCD product (seealso 15 in FIG. 1) is obtained at 41. It is then determined at 42whether any intermediate base is applicable to the CCD product. If not,then a direct transformation to the original base is applied to the CCDproduct at 43. The next CCD product is then obtained at 41, unless it isdetermined at 45 that all CCD products are co-registered, in which caseoperation is completed. If applicability of any intermediate base(s) isdetermined at 42, then a compound transformation is applicable. Thetransformation(s) to the intermediate base(s) is/are applied to the CCDproduct at 44, after which operation proceeds to 43, wheretransformation to the original base is applied, completing the compoundtransformation.

FIG. 5 diagrammatically illustrates an apparatus for producing a set ofco-registered CCD products according to example embodiments of thepresent work. A coarse registration unit 52 is configured to produce,from the set of raw SAR images 11, information 54 indicative of thetransformations required to register all of the SAR images to a base SARimage selected from the set 11. In some embodiments, the information 54includes only direct transformations to an original base image. In someembodiments, the information 54 includes both direct transformations andcompound transformations. A fine registration unit 13 is configured toproduce a set of CCD products 15 from the set of SAR images 11. Asmentioned above relative to FIG. 1, such production of CCD products isknown in the art. A transformation unit 56 is configured to produce aset of co-registered CCD products by suitably applying to the CCDproducts 15 the transformations indicated by information 54.

Although example embodiments of the present work are described above indetail, this does not limit the scope of the present work, which can bepracticed in a variety of embodiments.

What is claimed is:
 1. A method of producing a set of co-registeredcoherent change detection (CCD) products using an apparatus configuredto perform the method, the method comprising: providing a set oftemporally separated synthetic aperture radar (SAR) images of a targetscene; determining a plurality of transformations for respectivelytransforming a plurality of said SAR images to a predetermined imagecoordinate system using the apparatus; and using said transformations tocreate, from a set of CCD products produced from said set of SAR images,a corresponding set of co-registered CCD products using the apparatus,wherein said using includes applying said plurality of transformationsrespectively to a plurality of said CCD products, wherein one of saidtransformations is for transforming the associated SAR image directly tosaid predetermined image coordinate system, and wherein another of saidtransformations is a compound transformation for transforming theassociated SAR image to a further image coordinate system of a furthersaid SAR image, and further for transforming said further SAR image tosaid predetermined image coordinate system.
 2. The method of claim 1,wherein said compound transformation is for transforming the associatedSAR image to a still further image coordinate system of a still furthersaid SAR image, and further for transforming said still further SARimage to said further image coordinate system.
 3. An apparatus forproducing a set of co-registered CCD products, the apparatus comprising:an input for receiving a set of temporally separated SAR images of atarget scene; a SAR image registration unit coupled to said input andconfigured to determine a plurality of transformations for respectivelytransforming a plurality of said SAR images to a predetermined imagecoordinate system; and a transformation unit coupled to said SAR imageregistration unit and configured to use said transformations to create,from a set of CCD products produced from said set of SAR images, acorresponding set of co-registered CCD products, wherein saidtransformation unit is configured to apply said plurality oftransformations respectively to a plurality of said CCD products,wherein one of said transformations is for transforming the associatedSAR image directly to said predetermined image coordinate system, andwherein another of said transformations is a compound transformation fortransforming the associated SAR image to a further image coordinatesystem of a further said SAR image, and further for transforming saidfurther SAR image to said predetermined image coordinate system.
 4. Theapparatus of claim 3, wherein said compound transformation is fortransforming the associated SAR image to a still further imagecoordinate system of a still further said SAR image, and further fortransforming said still further SAR image to said further imagecoordinate system.
 5. A method of producing a set of co-registeredcoherent change detection (CCD) products using an apparatus configuredto perform the method, the method comprising: providing a set oftemporally separated synthetic aperture radar (SAR) images of a targetscene; determining a plurality of transformations for respectivelytransforming a plurality of said SAR images to a predetermined imagecoordinate system using the apparatus, wherein said predetermined imagecoordinate system is an image coordinate system of a selected said SARimage, wherein said determining includes registering said plurality ofSAR images to said selected SAR image, and wherein said selected SARimage is closest among said set of SAR images to a midpoint of a timeinterval in which said set of SAR images was captured; and using saidtransformations to create, from a set of CCD products produced from saidset of SAR images, a corresponding set of co-registered CCD productsusing the apparatus.
 6. The method of claim 5, wherein said timeinterval is six months, and said set of SAR images consists of 350 SARimages.
 7. The method of claim 5, wherein said using includes applyingsaid plurality of transformations respectively to a plurality of saidCCD products.
 8. A method of producing a set of co-registered coherentchange detection (CCD) products using an apparatus configured to performthe method, the method comprising: providing a set of temporallyseparated synthetic aperture radar (SAR) images of a target scene;determining a plurality of transformations for respectively transforminga plurality of said SAR images to a predetermined image coordinatesystem using the apparatus, wherein said predetermined image coordinatesystem is an image coordinate system of a selected said SAR image, andwherein said determining includes registering said plurality of SARimages to said selected SAR image; and using said transformations tocreate, from a set of CCD products produced from said set of SAR images,a corresponding set of co-registered CCD products using the apparatus,wherein said using includes applying said plurality of transformationsrespectively to a plurality of said CCD products, wherein one of saidtransformations is for transforming the associated SAR image directly tosaid image coordinate system of said selected SAR image, and whereinanother of said transformations is a compound transformation fortransforming the associated SAR image to a further image coordinatesystem of a further said SAR image, and further for transforming saidfurther SAR image to said image coordinate system of said selected SARimage.
 9. An apparatus for producing a set of co-registered CCDproducts, the apparatus comprising: an input for receiving a set oftemporally separated SAR images of a target scene; a SAR imageregistration unit coupled to said input and configured to determine aplurality of transformations for respectively transforming a pluralityof said SAR images to a predetermined image coordinate system, whereinsaid predetermined image coordinate system is an image coordinate systemof a selected said SAR image, wherein said SAR image registration unitis configured to determine said transformations by registering saidplurality of SAR images to said selected SAR image, and wherein saidselected SAR image is closest among said set of SAR images to a midpointof a time interval in which said set of SAR images was captured; and atransformation unit coupled to said SAR image registration unit andconfigured to use said transformations to create, from a set of CCDproducts produced from said set of SAR images, a corresponding set ofco-registered CCD products.
 10. The apparatus of claim 9, wherein saidtime interval is six months, and said set of SAR images consists of 350SAR images.
 11. The apparatus of claim 9, wherein said transformationunit is configured to apply said plurality of transformationsrespectively to a plurality of said CCD products.
 12. An apparatus forproducing a set of co-registered CCD products, the apparatus comprising:an input for receiving a set of temporally separated SAR images of atarget scene; a SAR image registration unit coupled to said input andconfigured to determine a plurality of transformations for respectivelytransforming a plurality of said SAR images to a predetermined imagecoordinate system, wherein said predetermined image coordinate system isan image coordinate system of a selected said SAR image, and whereinsaid SAR image registration unit is configured to determine saidtransformations by registering said plurality of SAR images to saidselected SAR image; and a transformation unit coupled to said SAR imageregistration unit and configured to use said transformations to create,from a set of CCD products produced from said set of SAR images, acorresponding set of co-registered CCD products, wherein saidtransformation unit is configured to apply said plurality oftransformations respectively to a plurality of said CCD products,wherein one of said transformations is for transforming the associatedSAR image directly to said image coordinate system of said selected SARimage, and wherein another of said transformations is a compoundtransformation for transforming the associated SAR image to a furtherimage coordinate system of a further said SAR image, and further fortransforming said further SAR image to said image coordinate system ofsaid selected SAR image.